Over the years we have tried, not very hard, to train our various German Shepherds to round up our flock. We've never had much success, they all do the same thing. Instead of gathering up the sheep into a nice bunch, they run straight into the middle, split them up and then take off after one of them.
It was easier to train the sheep to go where we wanted them!
I agree with FiB, a bucket works wonders.
Sharon